The collecting tubule of the kidney plays an important role in the excretion of dietary fixed acids produced by the liver. In certain disease states, the function of thc collecting tubule is impaired, which results in the retention by the body of dietary acids. The acid-base transport properties of individual cells in this nephron segment are poorly understood. Despite this, one finds repeated dogmatic statements in the literature based on poor data regarding the function of the cells in this segment. Previous studies have been hampered by the simple fact that the collccting duct possess at least 3 and possibly more cell types each of which likely have unique transport properties. In order to begin to unravel the mechanisms responsible for acid-base transport in individual cells in thc collecting duct we have developed a new inverted dual excitation confocal fluorcscence microscope coupled to the tubule perfusion apparatus. The feasibility of using this device to study acid-base transport in individual collecting duct cells is demonstrated in the preliminary studies. The hypotheses to be tested are: (1) whether the principal cells in the cortical, outer medullary and inner medullary collecting tubule play an important rolc in acid-base transport (2) whether the current models of the acid-base transport pathways in collecting duct intercalated cells are valid and 3) whether current cell models are applicable to both rat and rabbit. In addition, studies are proposed to uncover the mechanism(s) by which acid-base transport is modulated at the single cell level by mineralocorticoids, cAMP, PCO2, and metabolic acidosis. The submicron resolution of the proposed methodology will permit for the first time a detailed study of the apical and basolateral cellular transport processes involved in H+/base transport in individual cells of the cortical, outer medullary, and inner medullary collecting ducts. The results of these studies will make an important contribution to our evolving understanding of collecting duct acid-base physiology.